Phase-locked loop circuits are commonly used in circuits that require generation of a high-frequency periodic signal with the frequency of the signal being an accurate multiple of the frequency of a very stable and low-noise reference frequency signal. Phase-locked loop circuits also find applications where the phase of the output signal has to track the phase of the reference signal, thus the name phase-locked loop.
Phase-locked loop circuits are used for generating local oscillator signals in radio receivers and transmitters. The local oscillator signal is used for channel selection and thus is a multiple of a stable, low-noise and often temperature-compensated reference signal generator. Phase-locked loop circuits are also used for clock recovery applications in digital communication systems, disk-drive read-channels and the like. Phase-locked loop circuits are also used in frequency modulators and in the de-modulation of frequency-modulated signals. An overview of typical applications is discussed in "Monolithic Phase-Locked Loops and Clock Recovery Circuits, Theory and Design", Behzad Razavi, IEEE Press, 1996.
A typical phase-locked loop circuit includes a loop filter connecting a phase detector to a voltage-controlled oscillator. The loop filter defines the dynamics of the phase-locking feedback loop such that certain stability criteria are fulfilled and the loop doesn't enter an oscillatory condition. In second and higher order phase-locked loop circuits, this stabilization is commonly achieved by inserting a resistor into the loop filter. The resistor generates thermal noise that amounts to a contribution to the phase noise spectrum of the phase-locked loop output signal. The loop filter resistor noise can dominate the overall phase noise in the neighborhood of the loop bandwidth.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.